Computer program product and a node implementing power management by associated power management controllers based on distributed node power consumption and priority data

ABSTRACT

It is determined that a current node power consumption for a node is greater than a node power cap that defines a limit of power consumption for the node. Responsive to the current node power consumption being greater than the node power cap and until the current node power consumption is less than the node power cap, power reduction operations are performed. The power reduction operations comprise determining a power management zone of a plurality of power management zones having a lowest priority among the power management zones and having a power cap greater than a minimum power cap for the power management zone. The power reduction operations further comprise setting the power cap for the power management zone to a value less than a prior value assigned as the power cap for the power management zone.

BACKGROUND

Embodiments of the inventive subject matter generally relate to thefield of computing system power management, and, more particularly, todistributed power management of processor and memory systems.

Power consumption is an important factor in computing system design forreasons as diverse as maximizing hardware efficiency, increasingcomputing density and reducing capital costs. Much effort has been spentin reducing the amount of power consumed by a computing system. In fact,many computing systems include processors and hardware dedicated topower management. Some implementations can include centralized hardware,such as a single dedicated processor, that performs the power managementoperations. Some implementations include distributed power managementhardware, allowing simpler power management hardware to control a subsetof the system components, resulting in more responsive power management,greater efficiency, etc.

SUMMARY

Embodiments generally include a computer program product comprising acomputer readable storage medium having program instructions embodiedtherewith. The program instructions comprise program instructions todetermine that a current node power consumption for a node is greaterthan a node power cap that defines a limit of power consumption for thenode. The node comprises a plurality of power management zones and eachof the plurality of power management zones has a power cap that definesa limit of power consumption for the power management zone. The programinstructions further comprise program instructions to perform powerreduction operations in response to the current node power consumptionbeing greater than the node power cap and until the current node powerconsumption is less than the node power cap. The program instructionsfurther comprise program instructions to determine a power managementzone of the plurality of power management zones having a lowest priorityamong the plurality of power management zones and having a power capgreater than a minimum power cap for the power management zone. Theprogram instructions further comprise program instructions to set thepower cap for the power management zone of the plurality of powermanagement zones to a value less than a prior value assigned as thepower cap for the power management zone of the plurality of powermanagement zones.

Embodiments also generally include an apparatus comprising a pluralityof controllers. Each of the controllers is associated with a pluralityof components and each of the plurality of controllers is associatedwith a respective one of a plurality of power management zones. Theapparatus further comprises one or more computer readable storage mediahaving program instructions embodied therewith. The program instructionsare executable by a first of the plurality of controllers to cause theapparatus to determine that a current node power consumption for a nodeis greater than a node power cap that defines a limit of powerconsumption for the node. The program instructions are furtherexecutable by the first controller to perform power reduction operationsin response to a determination that the current node power consumptionbeing greater than the node power cap and until the current node powerconsumption is less than the node power cap. The program instructionsbeing executable by the first controller to cause the apparatus toperform the power reduction operations comprise program instructionsexecutable by the first controller to cause the apparatus to determinethat the power management zone associated with the first controller hasa lowest priority among the plurality of power management zones and hasa power cap greater than a minimum power cap for the power managementzone associated with the first controller. The program instructionsbeing executable by the first controller to cause the apparatus toperform the power reduction operations further comprises programinstructions executable by the first controller to cause the apparatusto set the power cap for the power management zone associated with thefirst of the plurality of power management controllers.

BRIEF DESCRIPTION OF THE DRAWINGS

The present embodiments may be better understood, and numerous objects,features, and advantages made apparent to those skilled in the art byreferencing the accompanying drawings.

FIG. 1 depicts a distributed power management system with a prioritizingpower cap analyzer and a workload analyzer.

FIG. 2 depicts a flowchart of example operations for setting a zonepower cap based on zone priorities.

FIG. 3 depicts a flowchart of example operations for determiningcomponent power caps based on workload characteristics.

FIG. 4 depicts an example computing system.

DESCRIPTION OF EMBODIMENT(S)

The description that follows includes exemplary systems, methods,techniques, instruction sequences and computer program products thatembody techniques of the present inventive subject matter. However, itis understood that the described embodiments may be practiced withoutthese specific details. For instance, although examples refer toprocessors and memory as example components, components can be anycomputing system component that includes, directly or indirectly, powerconsumption management functionality. In other instances, well-knowninstruction instances, protocols, structures and techniques have notbeen shown in detail in order not to obfuscate the description.

Terminology

For the following descriptions, a “node” includes a group of computingsystem components. Examples of a node include a server, a rack server, aboard with multiple microprocessors, a blade server, a multi-coreprocessor, etc. Examples of components include a processor, a memorymodule, an input/output hub controller, a processor core, core memory,etc. Some components can be nodes as well. For example, consider aserver with two multi-core processors. The server is a node comprisingthe two multi-core processors, which are components. The multi-coreprocessors are also nodes, each comprising multiple processor cores,which are components. Each processor core can be viewed as being acomponent of the server node as well.

A “power management zone” (hereinafter “zone”) is a collection ofcomponents that share the same power management controller (hereinafter“controller”). For example, a zone can include a processor and memoryassociated with the processor. Zones can be nested. For example, eachprocessor core might be a zone within a larger zone that includes theprocessor and memory associated with the processor. Whileimplementations can vary greatly, the descriptions below will use zonesthat include a single processor and memory associated with the processoras examples zones.

A controller can be implemented as any combination of hardware,software, or firmware. The term “controller” is merely used to describethe component or set of components that are responsible for implementingthe power management functionality and not to create a distinctionbetween a “controller” and a “processor”. In other words, while theterms “controller” and “processor” are used in the descriptions todescribe separate components, a controller can be a processor and viceversa. Further, the controller can be implemented using hardware,software, or firmware that implements other functionality in addition tothe power management functionality.

A “performance state” is a concept that describes the performance levelof a processor or processor core. The performance states (commonlyreferred to as “Pstates”) can be enumerated as P_(−m), . . . , P⁻¹, P₀,P₁, . . . , P_(n). In general, the lower the performance state, thelower the processor frequency and processor voltage, and thus, the lowerthe performance of the computing system. Although each performance statecorresponds to a certain frequency, the specific frequencies for a givenstate are implementation specific. For example, performance state P_(n)can correspond to the maximum processor frequency. Thus, performancestate P_(n) for a 2.5 gigahertz processor corresponds to 2.5 gigahertz,whereas performance state P_(n) for a three gigahertz processorcorresponds to three gigahertz. The scaling of frequency and voltagebetween performance states can also differ between implementations, evenif the maximum processor frequency is the same. For example, assume twocomputing systems have the same maximum processor frequency and definethe same number of performance states. A performance state change in oneimplementation can correspond to a frequency change of one hundredmegahertz, while a performance state change in the other implementationcan correspond to a frequency change of two hundred megahertz. Further,different implementations can enumerate performance states differently.For example, some implementations might not include negative performancestates (e.g., performance states are only enumerated as P₀, P₁, . . . ,P_(n)), some implementations might not include positive performancestates (e.g., performance states are only enumerated as P_(−m), . . . ,P⁻¹, P₀), some implementations might define performance states such thatthe highest performance state corresponds to the lowest performance ofthe computing system, etc. For the purposes of the discussion below, theperformance states will be defined as P₀, . . . , P₉, with P₀corresponding to the lowest computing system performance and P₉corresponding to the highest computing system performance.

Because processor frequency and voltage correspond to power consumption,performance states can act as a simple proxy for communicating whatlevel of performance/power consumption a particular processor orprocessor core is operating at. Performance states can apply to otherhardware as well. For example, performance state P₀ and P₁ cancorrespond to particular refresh rates of a memory module. Thedescriptions below will assume that power consumption levels arecommunicated using performance states, although other techniques (suchas communicating wattage directly) can be used.

Overview

Generally, computing systems have limits on the amount of power they canconsume, referred to as “power caps”. For example, a data center isgenerally designed to allow for a particular maximum amount of power tobe drawn by all systems in the data center. This power cap can bedivided among the various structures in the data center. For example,each rack can have its own power cap and each computing system in a rackcan have a power cap, where each power cap is a subset of the next powercap in the hierarchy. While the power caps can be divided unevenly(e.g., one computing system in a rack can have a higher power cap thananother) and the power caps can change dynamically, the total amount ofpower used by the computing systems in a particular rack generallycannot exceed the rack power cap.

The power caps can funnel down to the computing system components aswell. For example, a particular power outlet for a computing system in arack might have a maximum rating of sixty amperes (“amps”), limiting theamount of current that can be drawn by the computing system and thuslimiting the total amount of power that can be consumed. The power capon the computing system can then be split among the internal components.The power consumption of the internal components is typically managed bythe computing system's power management system.

Often times, power management systems are centralized—that is, a centralcontroller manages the power supplied to each component of the system.Centralized power management can be effective in controlling powerconsumption of the computing system, but can also be inefficient.Because the central controller manages power allocation for eachcomponent, the central controller is heavily occupied with managingpower consumption for the multiple components of the computing system.In computing systems where power allocation is modified frequently,power management uses resources computing the power allocations for thecomponents and incurs communication overhead for communicating the powerallocations throughout the computing system.

Distributing control and management of component power consumptionallows for scalability in a complex computing system with a multitude ofcomponents, as well as reducing processing overhead. The components aredivided into zones. Information about the power consumption of the zonesin a computing system can be collected and distributed to controllersfor each of the zones. This distribution of computing system wide powerconsumption information provides each controller with a node-level viewof power consumption.

By distributing the information to individual controllers, the workloadof calculating power caps for the zones is spread among the individualcontrollers. Each controller only calculates the power cap for theassociated zone, allowing each controller to be less complex than acentralized controller. By reducing controller complexity, thecontrollers can be more efficient and smaller, thus allowing theirplacement closer to the components being controlled (e.g., directly onthe processor die). Increasing the efficiency of the controllers andplacing them near the components allows for quicker reactions tochanging conditions, thus increasing performance and decreasing powerconsumption.

To facilitate calculation of the power caps of the zone components, thepower management system can include a distributor. The distributorcollates and distributes the node power cap and node power consumptionto the individual controllers. The distributor can determine the nodepower consumption by retrieving and summing the power consumption ofeach zone.

The individual controllers implement a set of operations that allow eachcontroller to calculate an updated power cap without communicating withthe other controllers. One technique that can be used to determine theupdated power cap is by maintaining proportionality among the zones. Forexample, consider a computing system with two zones, where one of thezones uses sixty percent of the available power and the other zone usesforty percent of the available power. If the power cap increases ordecreases, each controller determines what percentage of the currentpower consumption the associated zone consumes. Each controller thenincreases or decreases the zone power cap in the same proportion. Forexample, assume the node power cap is 1000 watts and the current powerconsumption is 900 watts. In the aforementioned two zone node, the firstzone uses 540 watts (sixty percent of the current power consumption) andthe second zone uses 360 watts (forty percent of the current powerconsumption). If the power cap subsequently increases to 1100 watts, thezone power caps increase proportionally to their power consumption.Thus, the first zone's power cap increases by sixty watts (sixty percentof the power cap increase) and the second zone's power cap increases byforty watts (forty percent of the power cap increase).

Such an implementation, however, results in the updated zone power capsbeing tied to the initial conditions present when the zone power capsare updated. Consider a scenario in which the first zone is running ahigh priority thread while the second zone is running a low prioritythread. If the power cap is changed, each zone adjusts its power cap asabove. However, the low priority thread might be less sensitive to powercap changes. In other words, the low priority thread may not benefitfrom increased performance. Thus, allotting any increase in the powercap to the second zone is inefficient, as the first zone can benefitfrom the increase in the power cap while the second zone cannot.Similarly, lowering the power cap to the high priority thread may resultin a larger performance decrease than lowering the power cap to the lowpriority thread.

Even in such an implementation, the power management system may correctitself after a few updates. For example, if the power cap is increasedfor both zones, the second zone, running the low priority thread, mightnot choose to consume more power in response to the increased power cap.The first zone, running the high priority thread, on the other hand,might consume all additional power available to it. This results in thefirst zone using a greater proportion of power, and thus given a greaterproportion of the power cap increases. A similar scenario presentsitself in the context of lowering the power caps. However, this delayedresponse is inefficient and does not fully correct the disparity. Inother words, zones that do not utilize power cap increases are givensmaller increases, but are still given increases that may not beutilized.

A distributed power management system can be implemented to remedy sucha result by utilizing a set of priority-based operations. In such animplementation, each zone is assigned a priority. When the power cap isupdated, the controller determines how to adjust the power cap based onthe priority of the zones. For example, assume, in the previous example,that the first zone is assigned a high priority and the second zone isassigned a low priority (coinciding with the priority of the runningthread, in this instance). If the node power cap decreases, thecontroller lowers the second zone's power cap prior to lowering thefirst zone's power cap. Thus, the high priority thread in the secondzone will maintain performance until the first zone's power cap islowered to a particular threshold.

A related issue occurs when the controller determines power caps for thecomponents within a zone. The differing tasks may result in differingloads on the components within a zone. For example, some tasks mayresult in a greater load on memory while other tasks result in a greaterload on a processor. When the characteristics of the tasks beingperformed in a zone changes, adjusting the power consumption of thecomponents proportional to usage can result in inefficiencies, similarto proportional adjustment of the zone power caps.

A distributed power management system can be implemented to adjustcomponent power consumption based on the characteristics of past,current, or prospective zone workloads. For example, a controller candetermine whether a current workload is processor-intensive ormemory-intensive. The controller can then adjust the power consumptionaccordingly. In other words, if the workload is processor-intensive, thecontroller can lower the power cap of the memory and increase the powercap of the processor. If the workload is memory-intensive, thecontroller can lower the power cap of the processor and increase thepower cap of the memory.

Distributed Power Management System with Performance and PowerBoundaries Example Illustrations

FIG. 1 depicts a distributed power management system with a prioritizingpower cap analyzer and a workload analyzer. A distributed powermanagement system 100 includes power management zone A 102A throughpower management zone N 102N. Power management zone (hereinafter “zone”)A 102A includes component A 104 and component B 108. Component A 104includes zone controller (hereinafter “controller”) 106. Controller 106includes a prioritizing power cap analyzer (hereinafter “power capanalyzer”) 110, workload analyzer 112, and power level selector 114. Thedistributed power management system 100 also includes a distributor 116.Each zone includes a zone controller (not depicted) similar to thecontroller 106.

Component A 104 and component B 108 can be any type of component thatincludes, directly or indirectly, the ability to modify the powerconsumption of the component. For example, component A 104 can be aprocessor and component B 108 can be memory. Components that do notinclude the ability to modify power consumption directly may beassociated with additional components that facilitate the ability tomodify the power consumption. For example, a component that does notinclude an integrated voltage regulator may be coupled with an externalvoltage regulator, allowing the voltage of the component to be modified.In some implementations, the controller 106 can be independent of othercomponents, instead of integrated with a component as depicted in FIG.1.

All zones, including zone B 102B through zone N 102N can be configuredsimilarly to zone A 102A or configured differently. For example, somezones may include different components or a different quantity of aparticular component (such as more or less memory). Further, thecomponents within the zones can function differently. For example, thecontroller 106 can be configured to receive global settings anddistribute them to controllers in the other zones. In other words, oneor more of the controllers can function as the distributor 116, insteadof the distributor 116 being a separate entity. Similarly, in someimplementations, the controller 106 and controllers in other zonescommunicate directly or through a communication network instead ofutilizing the distributor 116.

At stage A, the distributor 116 receives power consumption data from oneor more of the power management zones. Using the controller 106 as arepresentative controller, the distributor 116 can read the data fromthe controller 106 or the controller 106 can send the data to thedistributor 116. The data transfer can occur at regular intervals orirregular intervals. For example, instead of the distributor 116 readingthe data from the controller 106 every 250 microseconds, the controller106 can send the data to the distributor 116 whenever the data changes.The distributor 116 can communicate with the different zonessimultaneously or asynchronously. For example, the distributor 116 mayread data from the zones at the same time or one zone at a time. In someimplementations, the distributor 116 can communicate with components inzone A 102A other than the controller 106.

The data received from the controller 106 is not limited to powerconsumption data. The data can also include information such as thecurrent state of the zones, characteristics of the processes within thezones, and/or measurements and statistical information. For example, thedata can include the current power level of various components withineach zone, temperature data, assigned priorities, etc. In other words,the data can include any type of data that can be used by the othercontrollers to set the power levels. Data indicating priority for eachzone can be stored in the respective zone (such as in the controller forthe zone) or stored at the distributor 116. If priority data is storedlocal to the zones, the distributor 116 receives the priority data aswell. Further, priority data can be implicit. For example, the priorityof a particular zone might be determined based on the how much of thepower available to zone is being consumed. In other words, a zone thatis using all power available to the zone could have a higher prioritythan a zone using seventy-five percent of the power available to thezone, etc.

Zone priorities can be set in a variety of ways. For example, the zonepriorities can be configured by a user, such as an administrator. Thezone priority might be set as part of the computing system boot processand stored in persistent memory, allowing the zone priorityconfiguration to persist even after the computing system has been shutdown. Zone priorities might remain static during the operation of thecomputing system, or might be changed, depending on the implementation.Further, implicitly determined zone priorities can be subsequentlystored as explicit values. For example, the priority of a particularzone might be determined based on how much power available to the zoneis being consumed, as described above. However, instead of making thepriority determination when the priority data is used, the prioritydetermination might be made at other times and then saved as an explicitvalue.

After receiving the power consumption data from the controller 106, thedistributor 116 can calculate the total power consumption by summing thepower consumption data from all controllers. In some implementations,the power consumption data received from the controller 106 is brokendown by components and not just a single value, in which the powerconsumption of the individual components is summed. In someimplementations, the controllers calculate the total power consumption.Further, the node power consumption can include the power consumed byother components that are not part of the distributed power managementsystem 100, referred to as “background power”. Further, the distributor116 can perform other operations in addition to the collection of data,such as data validation, error correcting, etc.

At stage B, the distributor 116 distributes the data to the controllersin each zone. The distributor 116 can distribute the data to all of thecontrollers at once or to one controller at a time. The distributor 116can write the data to the controllers or the controllers can read thedata from the distributor 116. For example, the distributor 116 can sendan indication to the controller 106 that the data is available and, inresponse, the controller 106 can read the data from the distributor 116.Additional data can be distributed as well, such as global information.For example, all controllers rely on the node power cap to calculate thezone power caps. Thus, the distributor 116 can distribute the node powercap to the controllers.

At stage C, the power cap analyzer 110 uses the data received from thedistributor 116 at stage B to determine the power cap for zone A 102A.The power cap analyzer 110 can also use additional data, such aszone-specific data (workload information, temperature data, etc.) todetermine the power cap for zone A 102A. The specific analysis performedby the power cap analyzer 110 can vary between implementations based onthe data available to the power cap analyzer 110, thecomponents/configuration of zone A 102A, etc. Further, although depictedas occurring after stage B, the analysis performed at stage C can beperformed at any time, such as when conditions local to zone A 102Achange.

The power cap analyzer 110 first determines whether the node power caphas increased or decreased. Although some implementations may includethe distributor 116 distributing data to the controller 106 even if thedata has not changed from a previous distribution, the discussion hereinwill assume that if the data does not change, no changes are made to thecomponent settings. After determining whether the node power cap hasincreased or decreased, the power cap analyzer 110 determines whetherthe power cap for zone A 102A should be changed.

In the case that the node power cap has decreased and the node powerconsumption is still below the node power cap, changing the power capscan be optional. In other words, the zone power cap can be decreasedwhen, if ever, the node power consumption exceeds the node power cap.This allows the zones to maximize performance until the node powerconsumption actually surpasses the node power cap. Consider an exampleof a node with two zones, a 1000 watt node power cap and a 500 watt zonepower cap for each zone. Further assume that both zones are idling andeach draws 100 watts. Consider a scenario in which the node power capdrops to 800 watts. The controller for each zone could determine thatthe zone power cap should be lowered to 400 watts. Now assume that thefirst zone becomes active and begins running a high priority load,maxing out the power cap for the first zone. If the zone power caps werelowered to 400 watts, the performance of the first zone would be limitedto a level associated with a maximum power consumption of 400 watts.However, if the zone power caps were not lowered in response to thelower node power cap, the first zone would be limited to the performancelevel associated with a 500 watt power cap. Neither scenario results inthe node power cap being exceeded, while the second scenario allows forgreater performance. Because the second zone is still idled (onlydrawing 100 watts), the two scenarios result in node power consumptionof 500 watts and 600 watts, respectively, both of which are well withinthe node power cap. Thus, preemptively lowering the zone power caps canproduce inefficiencies. While preemptively lowering the zone power capsmay provide for a safer operating environment, scenarios in which nodepower consumption exceeds the node power cap are generally taken intoaccount during the design of the node and related hardware. However, insome implementations, if the sum of the zone power caps exceeds the nodepower cap by a certain amount or percentage, the zone power caps can belowered accordingly, as described below.

In the case in which the node power cap has decreased and the node powerconsumption exceeds the lowered node power cap, the power cap analyzer110 determines whether zones with a lower priority than zone A 102A havea power level that is at or below a particular threshold. For example,the power consumption/performance level of a particular zone can becommunicated using performance states. As described above, a particularperformance state can be defined as representing a minimum performancelevel before the zone components are idled (P₀ for this discussion).Thus, the power cap analyzer 110 would iterate through the dataassociated with the other zones. If one or more other zones have a lowerpriority and a performance state greater than P₀, the power cap analyzer110 can maintain the power cap for zone A 102A, allowing the lowerpriority zones to lower their power cap. If zone A 102A has the lowestpriority, the power cap analyzer 110 lowers the performance state ofzone A 102A. The power cap analyzer 110 can lower the performance stateincrementally or can determine the number of performance states that thepower cap analyzer 110 should lower zone A 102A by in order to reducethe node power consumption to below the node power cap. If zone A 102Aand one or more zones have the same priority and the priority is thelowest of all zones, all zones with the lowest priority can lower theirperformance state.

It may be the case that the lower priority zones cannot lower theirpower consumption enough to lower the node power consumption to belowthe node power cap. In some implementations, the power cap analyzer 110does not explicitly handle this case. For example, if the power capanalyzer 110 performs the analysis at regular intervals, the power capanalyzer 110 can simply keep performing the analysis until no additionallower priority zones can lower their zone power cap, thus forcing thepower cap analyzer 110 to lower its zone power cap. In someimplementations, the power cap analyzer 110 determines, based on theperformance states (i.e., power consumption) of the lower priorityzones, whether the lower priority zones can cover the excess powerconsumption. If not, the power cap analyzer 110 can lower the zone powercap immediately.

If the performance state of zone A 102A is already at the minimum level,the power cap analyzer 110 does nothing.

In the case that the node power cap has increased, the power capanalyzer 110 increases the power cap for zone A 102A. The zone power capcan be increased based on the priority of the zone, increasedproportionally based on the current power consumption, etc.

At stage D, the workload analyzer 112 determines characteristics of zoneA's workload. The workload analyzer 112 can use a variety of data todetermine the characteristics of the workload. For example, the workloadcan be associated with static metadata that indicates variouscharacteristics, such as metadata that indicates whether the workloadhas a greater impact on component A 104 or component B 108. Zone A 102Acan maintain statistics about the workload, such as how frequentlymemory is accessed. A large number of memory accesses can indicate thatthe workload is memory-intensive, for example. The workload analyzer 112can also look at the percentage of the component power caps used by therespective components. For example, if component A's power consumptionhas reached its power cap but component B's power consumption is underits power cap, the workload analyzer 112 can assume that the workloadcauses an increased load on component A 104. Further, the workloadanalyzer 112 is not limited to the current workload. The workloadanalyzer 112 can use historical data for past workloads or receive dataassociated with workloads that are scheduled to be run but are notcurrently running.

Once the workload analyzer 112 determines the characteristics of theworkload, the workload analyzer 112 determines how to adjust the powercaps of component A 104 and component B 108. For example, if theworkload analyzer 112 determines that the workload puts an increasedload on component A 104, the workload analyzer 112 might increase thepower cap for component A 104 but not for component B 108. In someinstances, the workload analyzer 112 might increase the power caps ofboth components an equal amount or an amount proportional to the amountof power consumed by each individual component. The workload analyzer112 can also take into account the interactions between the components.For example, if component B's power cap is too low, the performance ofcomponent B 108 may result in the appearance that the workload increasesthe load on component B 108 more than component A 104. For example,assume component A 104 is a processor and component B 108 is memory. Insuch a case, the processor might be waiting for data to load frommemory. Thus, while the processor might not be consuming all of theavailable power, once the data is loaded, the usage could spike. Assuch, instead of allocating all additional power to the memory, theworkload analyzer 112 could allocate some power to the memory and someto the processor. How each specific scenario is handled can vary betweenimplementations, as workloads typical to a particular implementationmight exhibit specific characteristics. For example, testing andanalysis might show that the workloads on a network server exhibitcertain characteristics in response to changing power caps, whileworkloads on a desktop used for graphic design exhibits differentcharacteristics. Thus, different implementations can be designed toadjust the power caps differently.

At stage E, the power level selector 114 determines what power level thecomponents should be set at. The prior analysis was mostly concernedwith power caps, which specify the power level that the entity governedby the power cap should not exceed. However, not all components willnecessarily consume all the power available to the components. In otherwords, the power consumption of a component will not always track thepower cap. For example, a four zone node power cap might be 1000 watts,but all zones of the node may be idling and thus only pulling 100 watts.Even if the total power consumption is 400 watts, the node power cap isstill 1000 watts. Thus, the power level selector 114 takes into accountrequests for other power levels while not allowing the power consumptionto go above the zone power cap. In other words, the power cap(s)determined by the power cap analyzer 110 and workload analyzer 112 mightbe only one of several different factors that determine the actual powerconsumption of a particular component.

FIG. 2 depicts a flowchart of example operations for setting a zonepower cap based on zone priorities.

At block 200, a power cap analyzer receives the node power consumptiondata, the node power cap data, the priority data for all zones, and thezone power consumption data for all zones. As described above, the powercap analyzer can receive the data by reading the data from one or morecomponents, such as a distributor or other zone controllers. Further,one or more other components can write the data to the power capanalyzer. The node power consumption data can be determined based on thezone power consumption data for all zones by summing the zone powerconsumption data. Thus, the power cap analyzer might not explicitlyreceive the node power consumption data, but rather calculate it fromother received data. Similarly, the power cap analyzer might not receivethe priority data explicitly, but might calculate the priority data fromthe data received, such as the zone power consumption data. Further, thepower cap analyzer might only receive a subset of the data at anyparticular time. For example, instead of receiving all data at once, thepower cap analyzer might only receive data that has changed. After thepower cap analyzer receives the node power consumption data, the nodepower cap data, the priority data for all zones and the zone powerconsumption data for all zones, control then flows to block 202.

At block 202, the power cap analyzer determines the local zone priority.To determine the priority of the local zone (i.e., the zone containingthe power cap analyzer), the power cap analyzer might read a valuestored within the zone. For example, the zone might include a registerthat stores a value representing the zone priority. The zone prioritymight be stored in another component, such as a distributor component.Thus, the power cap analyzer might receive the local zone priority withthe data received at block 200. If the zone priorities are calculatedbased on the data received, the power cap analyzer can determine thelocal zone priority in the same way as the priorities of the other zonesare determined. After the power cap analyzer determines the local zonepriority, control then flows to block 204.

At block 204, the power cap analyzer determines whether one or more zonepower caps are to be decreased. As discussed above, in some instancesthe zone power caps are not modified in response to a change in the nodepower cap. For example, if the node power consumption is still below thenode power cap, the zone power caps might not be decreased. Whether thezone power caps are to be decreased in a particular scenario can varybetween implementations. However, one or more zone power caps willtypically be decreased if the node power consumption is above the nodepower cap. If the power cap analyzer determines that one or more zonepower caps are to be decreased, control then flows to block 206. If thepower cap analyzer determines that no zone power caps are to bedecreased, the process ends.

At block 206, a loop begins in which the power cap analyzer iteratesthrough the data for each zone and determines whether a zone with alower priority than the local zone can have its zone power cap lowered.A first zone is selected as the current zone during the first passthrough block 206. On each additional pass through block 206, anotherzone becomes the current zone. Other data used to support the loop canbe initialized and/or updated. After the loop is initialized or updated,control then flows to block 208.

At block 208, the power cap analyzer determines whether the current zonehas a lower priority than the local zone. The priority of a zone can berepresented in a variety of ways. For example, if there are twopriorities (such as a low priority and a high priority), the prioritycan be represented by a single bit. Priorities can also be representedas scalar values, such as the integers zero through nine. Someimplementations might define lower values as having lower priority thanhigher values while some implementations might define higher values ashaving lower priority than lower values. For example, if the integerszero through nine are used to define the priorities, zero can either bethe lowest or the highest priority, depending on the implementation. Thepower cap analyzer can compare the local priority to the priority of thecurrent zone. If the priority of the current zone is lower than thepriority of the local zone, control then flows to block 209. If thepriority of the current zone is not lower than the priority of the localzone, control then flows to block 210.

At block 209, the power cap analyzer determines whether the currentzone's power cap can be lowered. As described above, a particular powercap level can be set as the minimum power cap for a zone. If the currentzone's power cap is at the minimum zone power cap, the current zone'spower cap cannot be lowered anymore. If the power cap analyzerdetermines that that current zone's power cap cannot be lowered, controlthen flows to block 210. If the power cap analyzer determines that thecurrent zone's power cap can be lowered, control then flows to block214.

Control flowed to block 210 if it was determined, at block 208, that thecurrent zone priority is not lower than the local zone priority. Controlalso flowed to block 210 if it was determined, at block 209, that thecurrent zone's power cap cannot be lowered. At block 210, the power capanalyzer determines whether all zones have been iterated over. In otherwords, the power cap analyzer determines whether all zones have been thecurrent zone. If not all zones have been iterated over, control thenflows back to block 206. If all zones have been iterated over, controlthen flows to block 212.

At block 212, the power cap analyzer sets the local zone's power cap toa lower power consumption level. As described above, the power capanalyzer can lower the local zone's power cap incrementally until thenode power consumption is lower than the node power cap (or the localnode's power cap is at the minimum power cap). In other words, the powercap analyzer can lower the local zone's power cap by a single incrementuntil the process is run again. The power cap analyzer can also lowerthe local zone's power cap based on the amount needed to bring the nodepower consumption below the node power cap at once. In other words, thepower cap analyzer can calculate the power level needed to bring thenode power consumption below the node power cap and set the local zone'spower cap to the calculated power level. If the calculated power levelis below the minimum power cap, the power cap analyzer can set the localzone's power cap to the minimum power cap. After the power cap analyzersets the local zone's power cap to a lower power consumption level,control then flows to block 214.

At block 214, the loop in which the power cap analyzer iterated over thezone data ends. At the end of the loop the power cap analyzer has eitherdetermined that there is one or more zones with a lower priority thanthe local zone that can also have their zone power cap lowered orlowered the local zone's power cap. After the loop in which the powercap analyzer iterated over the zone data ends, the process ends.

The operations for increasing the power caps in response to an increasein the node power cap using zone priorities are substantially similar tothe operations depicted in FIG. 2. For example, block 208 would bemodified to determine whether the current zone priority is higher thanthe local zone priority. Similarly, block 209 would be modified todetermine if the current zone's power cap can be increased. Further, asdescribed above, the various techniques used to adjust the zone powercaps can vary between implementations. For example, the power caps maybe lowered based on zone priorities while the power caps are increasedproportionally based on their power consumption.

FIG. 3 depicts a flowchart of example operations for determiningcomponent power caps based on workload characteristics.

At block 300, a workload analyzer receives an indication that a zoneworkload should be analyzed to determine characteristics of theworkload. The indication that the zone workload should be analyzed cantake a variety of forms. For example, a component within the zone mightnotify the workload analyzer anytime the workload changes, such as whena processor component begins executing a different thread. The workloadanalyzer might receive an indication after a particular time interval,thus permitting the workload analyzer to run at regular intervals. Theworkload analyzer might receive an indication that the node or zonepower cap has been updated. The specific indications that can triggeranalysis by the workload analyzer can vary between implementations.After the workload analyzer receives the indication that the zoneworkload should be analyzed to determine characteristics of theworkload, control then flows to block 302.

At block 302, the workload analyzer analyzes the workload to determinethe workload characteristics. The analysis can vary betweenimplementations. For example, the workload can be associated withmetadata that indicates the workload characteristics. For example, eachtime a processor begins executing a thread, one or more registers can bewritten to with metadata pertaining to the thread. The metadata mightindicate characteristics of the thread that are of interest to theworkload analyzer, such as whether the workload is processor-intensive,memory-intensive, I/O-intensive, etc. Such metadata can be set by theanalysis of one or more other components or set statically by adeveloper. The workload analyzer can analyze the operation of theworkload by analyzing statistics maintained by one or more othercomponents, as described above. The workload analyzer can also analyzehow much power each component is using while the workload is executing.After the workload analyzer analyzes the workload to determine theworkload characteristics, control then flows to block 304.

At block 304, the workload analyzer determines whether the workloadcharacteristics indicate that the component power caps should beadjusted. Because the zone has a single power cap that is divided amongthe zone's components, the component power caps form a ratio. The powercap ratio between the zone's components can be adjusted to provide thecomponent used most heavily by the workload with the greatestperformance. For example, a zone can include a processor and memory usedby the processor. If a workload is processor-intensive, the power capcan be divided between the processor and memory such that the componentpower cap ratio is weighted towards the processor. Thus, the processorwould get a larger power cap relative to the memory power cap, allowingthe processor to have increased performance. If the workload analyzerdetermines that the workload characteristics indicate that the componentpower caps should be adjusted, control then flows to block 306. If theworkload analyzer determines that that the workload characteristics donot indicate that the component power caps should be adjusted, theprocess ends.

At block 306, the workload analyzer adjusts the component power capratio to take advantage of the workload characteristics. For example, ifthe workload analyzer determined that the workload is bound to a firstcomponent, the workload analyzer might increase the power cap of thefirst component and decrease the power cap of a second component. Forexample, assume that the zone includes a processor and memory used bythe processor. Without analyzing the workload, the processor has a powercap of 170 watts and the memory has a power cap of 90 watts. Theworkload analyzer might subsequently determine that the workload is amemory-intensive workload. The workload analyzer might then change theprocessor power cap to 150 watts and increase the memory power cap to110 watts. Thus, the memory is afforded additional power, hopefullyensuring higher performance of the workload relative to the originalpower caps. After the workload analyzer adjusts the component power capratio, the process ends.

In some implementations, the possible node power cap values might notalign perfectly with the possible aggregate power consumption values. Inother words, the total amount of power consumed by all power managementzones (the aggregate power consumption) might not always equal the nodepower cap, even when each zone is using the maximum amount of poweravailable. This can occur due to the finite size of adjustments that canbe made to the components within the power management zones. In otherwords, the voltage and frequency of a processor is generally adjustedusing a finite number of steps. Thus, for example, the voltage of aprocessor might be adjusted in 1/10^(th) volt steps, while the frequencymight be adjusted in 250 megahertz steps. So, for example, it might bethe case that the node power cap is 1000 watts, while the aggregatepower consumption can only be 950 watts or 1050 watts. In such ascenario, a controller might set the zone power cap to a value thatwould result in an aggregate power consumption value of 950 watts (i.e.,the maximum increment that is below the node power cap). Further, insome implementations, there may be some acceptable variation in actualpower consumption that goes above the node power cap. For example, anaggregate power consumption of 1005 watts might be considered nearenough to a node power cap of 1000 watts that no action is taken.Specific tolerances can vary between implementations.

As will be appreciated by one skilled in the art, aspects of the presentinventive subject matter may be embodied as a system, method and/orcomputer program product. Accordingly, aspects of the present inventivesubject matter may take the form of an entirely hardware embodiment, anentirely software embodiment (including firmware, resident software,micro-code, etc.) or an embodiment combining software and hardwareaspects that may all generally be referred to herein as a “circuit,”“module” or “system.” Furthermore, aspects of the present inventivesubject matter may take the form of a computer program product embodiedin a computer readable storage medium (or media) having computerreadable program instructions embodied thereon.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Java, Smalltalk, C++ or the like,and conventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present inventive subject matter are described withreference to flowchart illustrations and/or block diagrams of methods,apparatus (systems) and computer program products according toembodiments of the inventive subject matter. It will be understood thateach block of the flowchart illustrations and/or block diagrams, andcombinations of blocks in the flowchart illustrations and/or blockdiagrams, can be implemented by computer program instructions.

These computer program instructions may be provided to a processor of ageneral purpose computer, special purpose computer, or otherprogrammable data processing apparatus to produce a machine, such thatthe instructions, which execute via the processor of the computer orother programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer program instructions may also bestored in a computer readable storage medium that can direct a computer,a programmable data processing apparatus, and/or other devices tofunction in a particular manner, such that the computer readable storagemedium having instructions stored therein comprises an article ofmanufacture including instructions which implement aspects of thefunction/act specified in the flowchart and/or block diagram block orblocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other device to cause aseries of operational steps to be performed on the computer, otherprogrammable apparatus or other device to produce a computer implementedprocess, such that the instructions which execute on the computer, otherprogrammable apparatus, or other device implement the functions/actsspecified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

FIG. 4 depicts an example computing system. A computing system 400includes a processor 401 (possibly including multiple processors,multiple cores, multiple nodes, and/or implementing multi-threading,etc.). The computing system 400 includes memory 407. The memory 407 maybe system memory (e.g., one or more of cache, SRAM. DRAM, zero capacitorRAM, Twin Transistor RAM, eDRAM, EDO RAM, DDR RAM, EEPROM, NRAM, RRAM,SONOS, PRAM, etc.) or any one or more of the above already describedpossible realizations of computer-readable media. The computing system400 also includes a bus 403 (e.g., PCI. ISA, PCI-Express,HyperTransport, InfiniBand®, NuBus, etc.), a network interface 405(e.g., an ATM interface, an Ethernet interface, a Frame Relay interface,SONET interface, wireless interface, etc.), and a storage device(s) 409(e.g., optical storage, magnetic storage, etc.). The computing system400 also includes a distributed power management system withprioritization and workload analysis 411. The distributed powermanagement system with prioritization and workload analysis 411 embodiesfunctionality to implement embodiments described above. The distributedpower management system with prioritization and workload analysis 411may include one or more functionalities that facilitate determiningpower caps based on priority data and workload analysis. Any one ofthese functionalities may be partially (or entirely) implemented inhardware and/or on the processor 401. For example, the functionality maybe implemented with an application specific integrated circuit, in logicimplemented in the processor 401, in a co-processor on a peripheraldevice or card, etc. Further, realizations may include fewer oradditional components not illustrated in FIG. 4 (e.g., video cards,audio cards, additional network interfaces, peripheral devices, etc.).The processor 401, the storage device(s) 409, and the network interface405 are coupled to the bus 403. Although illustrated as being coupled tothe bus 403, the memory 407 may be coupled to the processor 401.

While the embodiments are described with reference to variousimplementations and exploitations, it will be understood that theseembodiments are illustrative and that the scope of the inventive subjectmatter is not limited to them. In general, techniques for distributedpower management as described herein may be implemented with facilitiesconsistent with any hardware system or hardware systems. Manyvariations, modifications, additions, and improvements are possible.

Plural instances may be provided for components, operations orstructures described herein as a single instance. Finally, boundariesbetween various components, operations and data stores are somewhatarbitrary, and particular operations are illustrated in the context ofspecific illustrative configurations. Other allocations of functionalityare envisioned and may fall within the scope of the inventive subjectmatter. In general, structures and functionality presented as separatecomponents in the exemplary configurations may be implemented as acombined structure or component. Similarly, structures and functionalitypresented as a single component may be implemented as separatecomponents. These and other variations, modifications, additions, andimprovements may fall within the scope of the inventive subject matter.

What is claimed is:
 1. A computer program product comprising a computerreadable storage medium having program instructions embodied therewith,the program instructions comprising program instructions executable by afirst power management controller of a plurality of power managementcontrollers in a node to cause the first power management controller to:receive, from a distributor of the node, a current node powerconsumption for the node and a node power cap that defines a limit ofpower consumption for the node, the node power consumption based onpower consumption data from the plurality of power managementcontrollers, wherein the node comprises a plurality of components,wherein each of the plurality of components is associated with arespective one of a plurality of power management zones, wherein each ofthe plurality of power management zones is associated with a respectiveone of the plurality of power management controllers, and wherein thefirst power management controller is associated with only a first powermanagement zone of the plurality of power management zones; receive,from the plurality of power management zones or from the distributor,priority data indicating a priority for each of the plurality of powermanagement zones; determine that the current node power consumption isgreater than the node power cap; and in response to the determinationthat the current node power consumption is greater than the node powercap and until the current node power consumption is less than the nodepower cap, perform power reduction operations, wherein the programinstructions to perform the power reduction operations comprise programinstructions to cause the first power management controller to,determine that the first power management zone has a lowest priorityamong the plurality of power management zones based on the priority dataand has a zone power cap greater than a minimum power cap for the firstpower management zone; and set the zone power cap for the first powermanagement zone to a value less than a prior value assigned as the zonepower cap for the first power management zone.
 2. The computer programproduct of claim 1, wherein the program instructions further compriseprogram instructions executable by the first power management controllercause the first power management controller to: determine that the nodepower cap has increased and that the power caps for the plurality ofpower management zones, in the aggregate, are less than the node powercap; in response to the determination that the node power cap hasincreased and that the power caps for the plurality of power managementzones, in the aggregate, is less than the node power cap, perform powercap increase operations until the power caps for the plurality of powermanagement zones, in the aggregate, are equal to an aggregate powerconsumption value closest to the node power cap, wherein the programinstructions to perform the power cap increase operations compriseprogram instructions to, determine that the first power management zonehas a highest priority among the plurality of power management zones andhas a zone power cap less than a maximum power cap for the first powermanagement zone; and set the zone power cap for the first powermanagement zone to a value greater than a prior value assigned as thezone power cap for the first power management zone.
 3. The computerprogram product of claim 1, wherein the program instructions to performthe power reduction operations further comprise program instructions toassign a component power cap to each of the plurality of componentsassociated with the first power management zone based, at least in part,on the zone power cap for the first power management zone.
 4. Thecomputer program product of claim 1, wherein the program instructions toset the zone power cap for the first power management zone to the valueless than the prior value comprise program instructions to overwrite anindication of a first performance state with an indication of a secondperformance state, wherein the second performance state is associatedwith lower power consumption than the first performance state.
 5. Thecomputer program product of claim 1, wherein the program instructions toperform the power reduction operations further comprise programinstructions to assign a priority to the first power management zonebased, at least in part, on an amount of power consumed by the firstpower management zone with respect to the zone power cap of the firstpower management zone.
 6. A node, comprising: a plurality of components,wherein each of the plurality of components is associated with arespective one of a plurality of power management zones; a plurality ofpower management controllers, wherein each of the plurality of powermanagement controllers is associated with a respective one of theplurality of power management zones, the plurality of power managementcontrollers including at least a first power management controllerassociated with only a first power management zone of the plurality ofpower management zones; and one or more computer readable storage mediahaving program instructions embodied therewith, the program instructionsexecutable by the first power management controller to cause the firstpower management controller to, receive, from a distributor of the node,a current node power consumption for the node and a node power cap thatdefines a limit of power consumption for the node, the node powerconsumption based on power consumption data from the plurality of powermanagement controllers; receive, from the plurality of power managementzones or from a distributor of the node, priority data indicating apriority for each of the plurality of power management zones;determining that the current node power consumption is greater than thenode power cap; and in response to the determination that the currentnode power consumption is greater than the node power cap and until thecurrent node power consumption is less than the node power cap, performpower reduction operations, wherein the program instructions to causethe first power management controller to perform the power reductionoperations comprises program instructions to cause the first powermanagement controller to, determine that the first power management zonehas a lowest priority among the plurality of power management zones andhas a zone power cap greater than a minimum power cap for the firstpower management zone; and set the zone power cap for the first powermanagement zone to a value less than a prior value assigned as the zonepower cap for the first power management zone.
 7. The node of claim 6,wherein the program instructions are executable by the first powermanagement controller to further cause the first power managementcontroller to: determine that the node power cap has increased and thatthe power caps for the plurality of power management zones, in theaggregate, are less than the node power cap; in response to thedetermination that the node power cap has increased and that the powercaps for the plurality of power management zones, in the aggregate, isless than the node power cap, perform power cap increase operationsuntil the power caps for the plurality of power management zones, in theaggregate, are equal to an aggregate power consumption value closest tothe node power cap, wherein the program instructions to cause the firstpower management controller to perform the power cap increase operationscomprise program instructions to cause the first power managementcontroller to, determine that the first power management zone has ahighest priority among the plurality of power management zones and has azone power cap less than a maximum power cap for the first powermanagement zone; and set the zone power cap for the first powermanagement zone to a value greater than a prior value assigned as thezone power cap for the first power management zone.
 8. The node of claim6, wherein the plurality of components comprises a processor, whereinthe first power management controller comprises an on-chip controllerlocated on the processor.
 9. The node of claim 8, wherein the programinstructions to cause the first power management controller to performthe power reduction operations further comprise program instructions tocause the first power management controller to assign a component powercap to each of the plurality of components associated with the firstpower management zone based, at least in part, on the zone power cap forthe first power management zone.
 10. The node of claim 6, wherein theprogram instructions to cause the first power management controller toset the zone power cap for the first power management zone to the valueless than the prior value comprise program instructions to cause thefirst power management controller to overwrite an indication of a firstperformance state with an indication of a second performance state,wherein the second performance state is associated with lower powerconsumption than the first performance state.
 11. The node of claim 6,wherein the program instructions to cause the first power managementcontroller to perform the power reduction operations further compriseprogram instructions to cause the first power management controller toassign a priority to the first power management zone based, at least inpart, on an amount of power consumed by the first power management zonewith respect to the zone power cap of the first power management zone.